Nitrogen oxides are major environmental pollutants, particularly in those circumstances in which they are found in large quantities--for example, in power plant flue gases. SCR-type processes for reducing these oxides to non-polluting nitrogen have already gained commercial acceptance in a number of countries; e.g., Germany and Japan. And SCR processes are presently under consideration in the United States.
The stoichiometry of the SCR reaction is: EQU 4NH.sub.3 +4NO+O.sub.2 .fwdarw.4N.sub.2 +6H.sub.2 O
Commercially available SCR catalysts are V.sub.2 O.sub.5 -based. An example is V.sub.2 O.sub.5 /WO.sub.3 /TiO.sub.2. Other catalysts have also proven to be efficient under power plant flue gas conditions. These include H-form zeolites, transition metal oxides, and metal sulfates.
All of these catalysts have the drawback that the conversion must be carried out at a high--typically above 350.degree. C.--temperature. In commercial power plants, this requires that the catalytic reactor be installed in what is certainly a non-optimal location from the viewpoints of capital investment and maintenance costs; viz., between the economizer section and air preheater of the steam generator. As a consequence of the required high reaction temperature and reactor location, it is expensive to eliminate nitrogen oxides from the gases when these heretofore disclosed catalysts are employed.